Engine braking systems have been known and used for decades in conjunction with internal combustion engines, particularly diesel engines. Such systems include compression release brakes and exhaust brakes. These braking systems may be used alone or in combination with the other.
In simple terms, a compression release brake takes the load off the standard service brake by turning the internal combustion engine into a power-absorbing air compressor using a compression-release mechanism. When a compression release type brake is activated, the exhaust valves of one or more unfueled cylinders are opened near the top of the compression stroke. This releases the highly compressed air through the exhaust system with little energy returned to the piston. As the cycle repeats, the energy of the vehicle's forward motion (as transmitted through the vehicle's drive train to the engine) is dissipated, causing the vehicle to slow down.
In contrast, an exhaust brake uses exhaust back pressure within the engine to significantly increase braking power by restricting the flow of exhaust gases and increasing back pressure inside the engine. As used herein, engine exhaust back pressure is that pressure produced by the engine to overcome the hydraulic resistance of the engine's exhaust system in order to discharge the gases into the atmosphere. The increased backpressure in the engine creates resistance against the pistons, slowing the crankshafts rotation and helping to control the vehicle speed.
As known in the art, compression release and exhaust engine brakes can be used together to achieve substantial levels of braking power. Unfortunately, one of the disadvantages in a combination compression release and exhaust brake is high system loading seen by the overhead or valve train, i.e., those components that normally transmit valve actuation motions to the engine valves, such as cams, rocker arms, cam followers (roller or flat), etc. particularly during a transient event. An example of this is illustrated in FIG. 1.
In particular, FIG. 1 illustrates a control signal 104 used to control operation of an exhaust engine brake subsystem, another control signal 106 used to control operation of a compression release engine brake subsystem and a trace 102 illustrating the force applied to valve train components resulting from cylinder pressure as a function of time, measured in seconds. As known in the art, each peak illustrated in the trace 102 represents the peak forces applied to the valve train through each piston cycle of a given engine cylinder. When the control signals 104, 106 transition, in this example, from low to high to activate both the compression release and exhaust engine brake subsystems at approximately the same time, this results in a typical compression release transient peak 108, followed by an extended period of unusually high peaks 110 prior to normal, steady state operation 112. As a result of the period of excessively high loads 110, damage may be inflicted on the valve train or overhead.
Techniques that overcome these problems would represent a welcome advance in the art.